Fibers with longitudinal electrodes, i.e. electrodes that run parallel with the fiber core, have various applications. Primarily, such fibers are utilized when it is desired to affect the optical properties of the fiber core by means of the electro-optic effect. By having electrodes close to the fiber core, high electric fields can be produced in said core.
The idea of introducing longitudinal electrodes in an optical fiber has been proposed in the prior art. Typically, the electrodes are arranged in the cladding of the fiber, close to the fiber core.
In the prior art, electrodes in the form of thin wires have been introduced into longitudinal holes of an optical fiber. Such method is very time-consuming and cramped. Furthermore, since the wire electrodes have a diameter that is smaller than the diameter of the holes into which they are introduced, the wires will be curved within the holes. Consequently, the distance to the fiber core, or between two such wire electrodes, will vary along the fiber and therefore give rise to an uncertainty regarding the electrical field produced by an application of voltage to said electrodes.
Another proposed method of manufacturing optical fibers having longitudinal electrodes includes drawing a fiber from a preform comprising electrode material. This method is associated with at least one serious drawback. Namely, the thermal expansion coefficient and the melting point of the electrode material must be essentially the same as those of the fiber material (i.e. the glass composition of which the fiber itself is made). Of course, these restrictions limit the available options of applicable electrode materials.
It has also been proposed to introduce electrode material into a drawn fiber. In this case, the fiber is manufactured with longitudinal holes, running essentially parallel with the fiber core. Electrode material, such as liquid metal, is then sucked into these holes from one end by a vacuum at the opposite end of the fiber.
One method of filling liquid metal into longitudinal holes in an optical fiber is disclosed in the European Patent Application EP 0 308 114 A2. In this method, an optical fiber with spaces therein is used as a starting point. Said spaces are filled with molten metal by placing the fiber in a heated enclosure, with one end of the fiber in the liquid metal and the other end of the fiber in vacuum. The vacuum applied to the fiber then sucks metal into the spaces. When the fiber is cooled, the liquid metal in the spaces solidifies to form solid electrodes.
However, it has been found that the method described in EP 0 308 114 A2 is associated with some serious drawbacks. When the fiber containing the liquid metal is cooled in order for the metal to solidify, the difference in thermal expansion coefficient between the metal and the glass composition of the fiber may cause cracks to appear in the electrodes. Consequently, the electrical conductivity of the longitudinal electrodes is interrupted by such cracks. In other words, the electrodes are fragmented at the solidification process. Furthermore, voids may form in the liquid metal during the filling of the spaces in the fiber, which voids also give rise to discontinuities in the electrodes. In general, the fiber becomes brittle and fragile due to the internal stress.
In order to provide optical fibers having homogenous longitudinal electrodes of high quality, and to avoid brittleness of the fiber, new methods of introducing longitudinal electrodes into optical fibers are desired.